Surface light source device of side light type and liquid crystal display

ABSTRACT

A LCD comprises a surface light source device of side light type for auxiliary lighting. A fluorescent lamp is turned on to supply primary light to be introduce into a guide plate. A back face (upper face) of the guide plate provides a light control face including an emission promoting surface to promote emission from an illumination output face (lower face). The illumination output face provides a specular face without scattering power. Illumination light L 2  introduced into the guide plate escapes gradually from the back face and the illumination output face on the way of propagation. Some of illumination light emitted from the illumination output face reaches a reflection plate after transmitting a liquid crystal cell and polarization plates. Quantity of light reaching the reflection plate depends on directions of transmission polarization planes of first and second polarization plates as well as state of a liquid crystal layer (depending on voltage applied to transparent electrodes). The reflection plate reflects and scatters illumination output light. Some of thus reflected and scattered illumination output light is emitted out of the device after transmitting a polarization plate, the liquid crystal cell, the guide plate and another polarization plate in order, thereby contributing to displaying. The light control face (back face) is provided with emission promoting property to promote escaping of light L 2  from the illumination output face. This emission promoting property is provided so as to be once getting stronger according to distance from an incidence end face  25 A and to be followed by getting weaker. In another arrangement, a guide plate is disposed outside of a polarization plate in front of a liquid crystal cell or is interposed between the liquid crystal cell and another polarization plate on the back side thereof.

BACKGROUND

1. Field of Invention

The present invention relates to a surface light source device of sidelight type and a liquid crystal display, in particular, to a surfacelight source device of side light type which is applied to an auxiliaryillumination arrangement in a liquid crystal display and to a liquidcrystal display equipped with the light source device.

2. Related Art

Liquid crystal displays of a known type employ a transmission-typeliquid crystal and ones of another known type employ a reflection-typeliquid crystal. In general, the latter permits consumption of electricpower to be much saved as] compared with the former, because ambientlight can be utilized for image formation. And it is known that anauxiliary illumination device is mounted on the liquid crystal displayto enable the display to perform displaying under the condition of shortambient light such as at night.

So-called surface light source devices of the side light type aresuitable for such an application. This is because of their structuresuch that a primary light source and a guide plate are arranged side byside to add only a very small thickness to the liquid crystal display.It is known that a reflection-type liquid crystal may functionsupplementally as a transmission-type liquid crystal when a surfacelight source device of side light type applied to auxiliary illuminationis switched on.

FIG. 8 and FIG. 9 illustrate an example of conventional surface lightsource device of side light type which is structured according to theabove manner. Referring to FIGS. 8 and 9, a liquid crystal display 1comprises a surface light source device of side light type arrangedbehind a liquid crystal display panel 2. That is, the surface lightsource device 3 provides an auxiliary illumination means. The liquidcrystal display 1 is viewed from above FIGS. 8 and 9.

The liquid crystal display panel 2 comprises a transmission-reflectionplate 5, a polarization plate 6, a glass substrate 7, a liquid crystallayer 8, another glass substrate 9 and another polarization plate 10which are disposed laminatedly in order. In these members, thetransmission-reflection plate 5 is located at the closest position tothe surface light source device 3.

Transparent electrodes are formed on surfaces of the glass substrates 7and 9, respectively, providing transparent electrode patterns betweenwhich a liquid crystal material is interposed and sealed to form theliquid crystal layer 8.

The liquid crystal display panel 2 provides liquid crystal cells withmatrix-like arrangement and each of the cells rotates a polarizationplane of light transmitting therethrough depending on voltage applied tothe transparent electrodes.

In general, the polarization plates 6 and 10 are orientated so that thetransmission polarization plane of the polarization plate 6 is eitherperpendicular or parallel to that of the polarization plate 10.

Each of the polarization plates 6 and 10 permits a componentcorresponding to a certain direction of polarization plane (transmissionpolarization plane) to transmit exclusively.

The transmission-reflection plate 5 is a member provided with bothtransmissivity and reflectivity, being arranged so that scattering powerand high reflectance is demonstrated with respect to light which istransmitted through the liquid crystal cells while being demonstratedhigh transmissivity with respect to light which is supplied from thesurface light source device 3.

On impinging of ambient light L1 to the liquid crystal display panelfrom above in FIG. 9, some component of the ambient light transmitsthrough the polarization plate 10, the glass substrate 9, the liquidcrystal layer 8, the glass substrate 7 and the polarization plate 6 toreach the transmission-reflection plate 5.

Quantity of light reaching the transmission-reflection plate 5 dependson factors including directions of transmission polarization planes ofthe polarization plates 6, 10 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes).

The transmission-reflection plate 5 scatters and reflects ambient lightL1. However, it involves partial transmission therethrough, because thetransmission-reflection plate 5 is provided with transrnissivity aswell.

Some component of the scattered and reflected light transmits throughthe polarization plate 6, the glass substrate 7, the liquid crystallayer 8, the glass substrate 9 and the polarization plate 10 in order,being emitted toward the outside to contribute to displaying.

Quantity of light emitting toward the outside varies depending onfactors including directions of transmission polarization planes of thepolarization plates 10, 6 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes). Accordingto this principle, bright-dark distribution is formed to provide imageto be viewed.

The surface light source device 3 is switched on usually when ambientlight is short. In the illustrated arrangement, the surface light sourcedevice 3 functions as a back-lighting means. The surface light sourcedevice 3 comprises a guide plate 12 and a primary light source disposedbeside the guide plate.

The primary light source 13 is composed of, for instance, a fluorescentlamp (cold cathode lamp) 14 and a reflector 15 backing the lamp. Whenthe fluorescent lamp 14 is turned on, primary light is introduced intothe guide plate 12 through an opening of the reflector 15 and an endface 12A of the guide plate. The guide plate 12 is, for example, made ofa transparent resin such as acrylic resin. Alternatively, a scatteringguide plate having scattering power inside is employed occasionally.

In the illustrated case, the guide plate 12 has a roughened back face(the lower face in FIG. 9). Illumination light L2 introduced into theguide plate 12 is diffused and reflected at the back face and is emittedfrom an illumination output face (the upper face in FIG. 9) on the wayof propagation. A reflection member (not shown) is disposed along theback face of the guide plate 12 occasionally.

As required, an additional member such as a prism sheet or a lightdiffusion sheet is disposed along the illumination output face of theguide plate 12 in order to modify directivity of emission.

Some of the outputted illumination light from the surface light sourcedevice 3 transmits through the transmission-reflection plate 5. Thensome component of it is emitted toward the outside after transmittingalong paths (polarization plate 6→glass substrate 7→liquid crystal layer8→glass substrate 9→polarization plate 10) similar to those of ambientlight L1, thereby contributing to displaying.

Quantity of light emitting toward the outside varies, in the same manneras that of ambient light L1, depending on factors including directionsof transmission polarization planes of the polarization plates 10, 6 andstate of the liquid crystal layer 8 (depending on voltage applied to thetransparent electrodes). According to this principle, bright-darkdistribution is formed to provide image to be viewed.

A problem with the above-described conventional liquid crystal display 1is that ambient light is utilized at a low efficiency. This is becausethe transmission-reflection plate 5 disposed at the back side of thepolarization plate 6 for auxiliary illumination has transmissivity whichallow some of ambient light L1 to be leaked out. Needless to say, thisleakage leads to reduction in quantity of light contributing todisplaying, thereby lowering contrast of display. In particular, colorimage displaying LCDs will suffer from an inferior display qualitybrought by short brightness and low contrast.

OBJECT AND SUMMARY OF INVENTION

The present invention has been proposed under the aforesaid background.An object of the present invention is to provide a liquid crystaldisplay, which operates to display images with high brightness and highcontrast as well as with small photo-energy loss, and a surface lightsource device of side light type to be mounted on the display.

A surface light source device of side light type in accordance with thepresent invention is applied to auxiliary lighting crystal displaycomprising a liquid crystal display panel. The surface light sourcedevice comprises a guide plate and a primary light source supplyingprimary light to an incidence end face, which is provided by a minorface of the guide plate. Major faces of the guide plate provides anillumination output face and a back face.

According to a feature of the present invention, the illumination outputface of the guide plate has non-scattering property and performs lightsupply light to a front face of the liquid crystal display panel. Andthe back face of the guide plate provides a light control face havingemission promoting property which helps light propagating within theguide plate to be escaped from the illumination output face. Thisemission promoting property preferably tends to be more intensiveaccording to distance from the incidence end faces of the guide plate.

In a typical embodiment, the light control face includes a great numberof fine regions to promote emission. The fine regions are preferablyarranged with irregularity. The fine regions may be roughened regionspartially occupying the back face. Each fine region is preferablyprovided with almost invisible dimension.

The surface light source device of side light type, which may bevariously embodied as above, can be arranged so that output light fromthe illumination output face is supplied toward the front of a liquidcrystal display panel, providing a liquid crystal display in accordancewith the present invention.

Besides, the present invention provides a liquid crystal displayequipped with the surface light source device of side light typeaccording to an improved arrangement. The liquid crystal displaycomprises a liquid crystal display panel and a surface light sourcedevice of side light type for auxiliary illumination, the panelincluding a crystal layer and a polarization plate disposed at the frontside of the liquid crystal layer.

The surface light source device of side light type comprises a guideplate and a primary light source supplying primary light to an incidenceend face which is provided by a minor face of the guide plate, the guideplate having major faces to provide a back face and an illuminationoutput face. And the guide plate is interposed between the liquidcrystal layer and the polarization plate to provide an improvedarrangement so that the illumination output face is directed to theliquid crystal layer.

The illumination output face of the guide plate has non-scatteringproperties while the back face of the guide plate provides a lightcontrol face having emission promoting properties which help lightpropagating within the guide plate to escape from the illuminationoutput face. The guide plate may be provided with “non-scatteringproperties” and “emission promoting properties” according to otherstyles. The back may be provided with “non-scattering properties” whilethe illumination output face may be provided with “emission promotingproperties”.

Such an improved arrangement also allows the liquid crystal display tohave “emission promoting properties” embodied in various manners.

First, these emission promoting properties preferably tend to be moreintensive according to distance from the incidence end faces of theguide plate. Next, the light control face may include a great number offine regions to promote emission. The fine regions are preferablyarranged with irregularity. The fine regions may be roughened regionspartially occupying the back face. Each fine region is preferablyprovided with almost invisible dimensions.

The aforementioned and other features of the present invention will beunderstood in more detail from the following description with theaccompanied drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1, is an exploded perspective view of a liquid crystal display inaccordance with an embodiment of the present invention;

FIG. 2 is a cross section view along line B—B indicated in FIG. 1;

FIG. 3a is a plan view illustrating a light control pattern formed on aguide plate in order to promote emission;

FIG. 3b is a graph to show a plotting of covering rate of the lightcontrol pattern;

FIG. 4 is a graph to show directional characteristics of emission whichis emitted from the back face and contributes to displaying;

FIG. 5 is a graph to show directional characteristics of emission whichis directly emitted from the back face;

FIG. 6 is a cross section view of a liquid crystal display in accordancewith another embodiment of the present invention;

FIG. 7 is a cross section view of a liquid crystal display in accordancewith yet another embodiment of the present invention;

FIG. 8 is an exploded perspective view of a conventional liquid crystaldisplay which employs a reflection-type liquid crystal; and

FIG. 9 is a cross section view along line C—C indicated in FIG. 8.

EMBODIMENTS

Referring to FIGS. 1 and 2, illustrated is a liquid crystal display inaccordance with an embodiment of the present invention. Elements used incommon to the arrangement shown in FIG. 8 or FIG. 9 are indicated bycommon references, with repeated descriptions being simplified.

A liquid crystal display 20 comprises a surface light source device ofside light type 22 disposed in front of (i.e. at the viewing side of) aliquid crystal display panel 21. That is, the surface light sourcedevice 22 provides an auxiliary front-lighting unit. The liquid crystaldisplay is viewed from above in FIGS. 1 and 2.

The liquid crystal display panel 21 comprises a reflection plate 23, afirst polarization plate 6, a glass substrate 7, a liquid crystal layer8, another glass substrate 9 and a second polarization plate 10 whichare disposed laminatedly in order. In these members, the secondpolarization plate 10 is located at the closest position to the surfacelight source device 22.

Transparent electrodes are formed on surfaces of the glass substrates 7and 9, respectively, providing transparent electrode patterns betweenwhich a liquid crystal material is interposed and sealed to form theliquid crystal layer 8.

The liquid crystal display panel 21 provides liquid crystal cells withmatrix-like arrangement and each of the cells rotates a polarizationplane of light transmitting therethrough depending on voltage applied tothe transparent electrodes.

Each of the polarization plates 6 and 10 permits a componentcorresponding to a certain direction of polarization plane (transmissionpolarization plane) to transmit exclusively.

In general, the polarization plates 6 and 10 are orientated so that thetransmission polarization plane of the first polarization plate 6 iseither perpendicular or parallel to that of the second polarizationplate 10.

The reflection plate 23 is a member provided with scattering propertiesand high reflectivity with respect to light which is transmitted throughthe liquid crystal cells, being produced, for example, by applyingvapor-evaporation of a metal material such as Ag or Al, onto a roughenedsurface of the substrate of the reflection plate 23. This reflectionplate 23 is employed instead of the transmission-reflection plate 5,which is employed in the arrangement shown in FIGS. 8 and 9, and has notransmissivity.

On impinging of ambient light L1 to the liquid crystal display panelfrom above in FIG. 2, some component of the ambient light transmitsthrough the polarization plate 10, the glass substrate 9, the liquidcrystal layer 8, the glass substrate 7 and the polarization plate 6 toreach the reflection plate 23.

Quantity of light reaching the reflection plate 23 depends on factorsincluding directions of transmission polarization planes of thepolarization plates 6, 10 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes).

The reflection plate 23 scatters and reflects ambient light L1.Substantially no light transmits through the reflection plate 23. Somecomponent of the scattered and reflected light transmits through thepolarization plate 6, the glass substrate 7, the liquid crystal layer 8,the glass substrate 9 and the polarization plate 10 in order, beingemitted toward the outside to contribute to displaying.

Quantity of light emitting toward the outside varies depending onfactors including directions of transmission polarization planes of thepolarization plates 10, 6 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes).

According to this principle, bright-dark distribution is formed toprovide images to be viewed.

The surface light source device 22 is switched on usually when ambientlight is short. The surface light source device 22 comprises a guideplate 25 and a primary light source 13 disposed beside the guide plate.

The primary light source 13 is composed of, for instance, a fluorescentlamp (cold cathode lamp) 14 and a reflector 15 backing the lamp. Whenthe fluorescent lamp 14 is turned on, primary light is introduced intothe guide plate 25 through an opening of the reflector 15 and an endface 25A of the guide plate. The guide plate 25 is, for example, made ofa transparent resin such as acrylic resin.

In the present embodiment, a back face (the upper face in FIG. 2) 25B ofthe guide plate 25 provides a light control face 25D which includes anemission promoting face helping emission from an illumination outputface 25C. Details of the light control face 25D is described later. Onthe other hand, the illumination output face 25C is a specular facehaving substantially no scattering power.

Illumination light L2 introduced into the guide plate 25 goes back andforth repeatedly between the back face 25B (light control face 25D) andthe illumination output face 25C on the way of propagation. This processinvolves gradual emitting from the illumination output face 25C.

Some of the outputted illumination light from the illumination outputface 25C transmits through the polarization plate 10, the glasssubstrate 9, the liquid crystal layer 8, the glass substrate 7 and thepolarization plate 6, reaching the reflection plate 23.

The quantity of light reaching the reflection plate 23 depends onfactors including directions of transmission polarization planes of thepolarization plates 10, 6 and the state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes).

The reflection plate 23 scatters and reflects the outputted illuminationlight. Substantially no light transmits through the reflection plate 23.Some components of the scattered and reflected light transmits throughthe polarization plate 6, the glass substrate 7, the liquid crystallayer 8, the glass substrate 9, the polarization plate 10 and the guideplate 25 in order, being emitted toward the outside to provide light L2Ato contribute to displaying.

The quantity of light emitting toward the outside varies depending onfactors including directions of transmission polarization planes of thepolarization plates 10, 6 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes). Accordingto this principle, bright-dark distribution is formed to provide imagesto be viewed.

The light control face 25D is provided with emission promotingproperties to promote emission of illumination light L2. These emissionpromoting properties tend to be at first stronger according to distancefrom the incidence face 25A and to be weaker thereafter. The reason whyweaker emission promoting properties are assigned around a distal end isthat illumination output is generally increased around the distal end asa distal end face brings reflection light thereabout.

Emission promoting regions are distributed on the back face 25Baccording to a light control pattern. The light control pattern isdesigned so that the above tendency is realized. An example of the lightcontrol pattern according to the present invention is illustrated inFIG. 3a.

Referring to FIG. 3a, each emission promoting region on the back face25B has a shape like a fine dot. A great number of fine dots aredistributed so that covering rate (covering area per unit area) tends tobe at first increased according to distance from the incidence face 25Aand to be reduced thereafter, as shown in FIG. 3b. Such distributionrealizes the aforesaid tendency.

Each emission promoting region consists of, for instance, a finemat-processed region. Size of the dot-like fine region is so small as tobe invisible if observation is attempted from the back face side. It isalso preferable that this size is smaller than the structural period ofthe liquid crystal cells. Dot size (diameter) is practically less than80 μm and is preferably, in particular, less than 35 μm.

FIG. 3b shows a curve illustrating the covering of dots according todesign. According to a manner of design, first and foremost, a guideplate 25 without light control pattern 27 is measured for intensitydistribution of light going toward the frontal direction with respect tothe illumination output face 25C. The emission promoting face modifiesthis intensity distribution and improves the output efficiency ofillumination.

Under considerations of the measured intensity distribution, variationof the covering rate is calculated so that a desirable intensitydistribution is realized. In a typical case, approximately uniformdistribution of illumination is desired. Then, a grid is set at pitchesvarying according to the calculated changing of covering rate.

An example of such a grid is shown with broken lines in FIG. 3a. Aconstant number of (for example, one) dots (fine emission promotingregions) are allotted and arranged for each grid cell. Dot arrangementis determined preferably so that no periodic regularity appears. Suchirregular dot arrangement prevents Moire fringes which might begenerated in connection with a fine periodic structure of the liquidcrystal cells.

The peak (the maximum covering rate) in the graph of FIG. 3b preferablydoes not mark an excessively high score. This is because excessivelyhigh covering rate will increase direct escape from the back face 25Btoward the frontal direction (toward the upper direction of FIG. 2) andwill thereby impede the illumination promotion function for promotingemission from the illumination output face 25C.

Besides, high covering rate will effect strong light diffusion whichmight make the displayed image blurred. From such a viewpoint, it ispractical to employ a design such that the maximum covering rate is notlarger than 50%, and it is preferably that the maximum covering rate isnot larger than 20%, for example no larger than 10%.

Referring to FIGS. 4 and 5, directional characteristics of emission fromthe back face 25B of the guide plate 25 shown in FIG. 3, which isrepresented by a beam L2A and contributes to displaying, is plotted(FIG. 4); and directional characteristics of emission which is directlyemitted from the back face 25B of the guide plate 25 shown in FIG. 3 isplotted (FIG. 5). The horizontal axis represents angles for directionindication and corresponds to y θ in FIG. 2. The vertical axisrepresents intensity of emission toward a direction of angle y θ.

From comparison of both graphs, it is understood that the former marksmuch higher scores. Direct emission L2B from the back face 25B giveswhitish background to the overall displayed image. Thereby the displayedimage looks as if black-level portions are floating on the whitebackground. This allows the present embodiment to provide display withhigh contrast and brightness.

Further, comparing the present embodiment with the prior arts shown inFIGS. 8 and 9, either ambient light or output light of the surface lightsource device is utilized for displaying image at an improvedefficiency. This is because the reflection plate 23 does not causeambient light to be leaked and output light of the surface light sourcedevice is inputted into the liquid crystal cells through no elementhaving a low transmissivity (such as the reflection-transmission plate 5in FIGS. 8 and 9).

Referring to FIG. 6, illustrated is a liquid crystal display inaccordance with the another embodiment of the present invention.Elements used in common to the arrangements shown in FIG. 8, FIG. 9,FIG. 1 or FIG. 2 are indicated by common references, withrepeated-descriptions being simplified.

A liquid crystal display 40 comprises a surface light source device ofside light type disposed in front of (i. e. at the viewing side of) aliquid crystal layer 8. The present embodiment employs an arrangementsuch that the surface light source device is incorporated in a liquidcrystal display panel to provide an auxiliary front-lighting means.Display screen is viewed from above in FIGS. 6.

In order from the back side (from the bottom in FIG. 6), laminatedlydisposed are a reflection plate 23, a first polarization plate 6, aglass substrate 7, a liquid crystal layer 8, a glass substrate 9, aguide plate 25 and a second polarization plate 10. An important featureof the arrangement is that the guide plate 25 is interposed between theglass substrate 9 and the second polarization plate 10.

Transparent electrodes are formed on surfaces of the glass substrates 7and 9, respectively, providing transparent electrode patterns betweenwhich a liquid crystal material is interposed and sealed to form theliquid crystal layer 8. Each liquid crystal cell rotates a polarizationplane of light transmitting therethrough depending on voltage applied tothe transparent electrodes.

Each of the polarization plates 6 and 10 permits a componentcorresponding to a certain direction of polarization plane (transmissionpolarization plane) to transmit exclusively. In general, thepolarization plates 6 and 10 are orientated so that the transmissionpolarization plane of the first polarization plate 6 is eitherperpendicular or parallel to that of the second polarization plate 10.

The reflection plate 23 is a member provided with scattering propertyand high reflectivity with respect to light which is transmitted throughthe liquid crystal cells, having substantially no light transmissivity,which may be the same element as is employed in the previous embodiment.

On impinging of ambient light L1 to the liquid crystal display panelfrom above in FIG. 6, some component of the ambient light transmitsthrough the polarization plate 10, the guide plate 25, the glasssubstrate 9, the liquid crystal layer 8, the glass substrate 7 and thepolarization plate 6 to reach the reflection plate 23.

Quantity of light reaching the reflection plate 23 depends on factorsincluding directions of transmission polarization planes of thepolarization plates 6, 10 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes).

The reflection plate 23 scatters and reflects ambient light L1.Substantially no light transmits through the reflection plate 23. Somecomponent of the scattered and reflected light transmits through thepolarization plate 6, the glass substrate 7, the liquid crystal layer 8,the glass substrate 9, the guide plate 25 and the polarization plate 10in order, being emitted toward the outside to contribute to displaying.

Quantity of light emitting toward the outside varies depending onfactors including directions of transmission polarization planes of thepolarization plates 10, 6 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes). Accordingto this principle, bright-dark distribution is formed to provide imageto be viewed.

The surface light source device is switched on usually when ambientlight is short. The surface light source device 22 comprises a guideplate 25 and a primary light source 13 disposed beside the guide plate.

The primary light source 13 is composed of, for instance, a fluorescentlamp (cold cathode lamp) 14 and a reflector 15 backing the lamp. Whenthe fluorescent lamp 14 is turned on, primary light is introduced intothe guide plate 25 through an opening of the reflector 15 and an endface 25A of the guide plate. The guide plate 25 may be the same asemployed in the previous embodiment.

That is, a back face (the upper face in FIG. 6) 25B of the guide plate25 provides a light control face 25D which includes an emissionpromoting face for helping emission from an illumination output face25C. The illumination output face 25C is a specular face havingsubstantially no scattering power.

Illumination light L2 introduced into the guide plate 25 goes back andforth repeatedly between the back face 25B (light control face 25D) andthe illumination output face 25C on the way of propagation. This processinvolves gradual emitting from the illumination output face 25C.

Some of the outputted illumination light from the illumination outputface 25C transmits through the glass substrate 9, the liquid crystallayer 8, the glass substrate 7 and the polarization plate 6, reachingthe reflection plate 23.

Quantity of light reaching the reflection plate 23 depends on factorsincluding directions of transmission polarization planes of thepolarization plates 10, 6 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes).

The reflection plate 23 scatters and reflects the outputted illuminationlight. Substantially no light transmits through the reflection plate 23.Some component of the scattered and reflected light transmits throughthe polarization plate 6, the glass substrate 7, the liquid crystallayer 8, the glass substrate 9, the guide plate 25, the polarizationplate 10 in order, being emitted toward the outside to provide light L2Acontribute to displaying.

The quantity of light emitting toward the outside varies depending onfactors including directions of transmission polarization planes of thepolarization plates 10, 6 and state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes). Accordingto this principle, bright-dark distribution is formed to provide imagesto be viewed.

The light control face 25D is provided with emission promotingproperties to promote emission of illumination light L2. These emissionpromoting properties tend to be at first stronger according to distancefrom the incidence face 25A and to be weaker thereafter. The reason whysuch reduced emission promoting properties are assigned to around adistal end is that a distal end face produces reflection light whichwill cause illumination output to be increased.

The present embodiment also employs emission promoting regions which aredistributed on the back face 25B according to a light control pattern.The light control pattern is designed so that the above tendency isrealized. The light control pattern illustrated in FIG. 3a may beemployed in the present embodiment, too. Repeated descriptions on thelight control pattern shown in FIG. 3a are omitted.

Comparing the present embodiment with the embodiment of FIG. 1, thefollowings will be understood. Any one of the embodiments do not succeedin perfect suppression of illumination light L2B which is directlyemitted from the back face 25B of the guide plate 25.

However, the present embodiment nevertheless prevents a half of suchillumination light L2B from escaping to the outside because thepolarization plate 10 is arranged at the outside of the guide plate 25.Accordingly, white-ish background leading to reduced display of contrastis less conspicuous compared with the case of the embodiment of FIG. 1.

It is to be further noted that some of illumination light L2A emittedfrom the illumination output face 25C is emitted from the back face 25Bafter being reflected by an element such as the glass substrate 9 andpenetrating through the guide plate 25. Such components fail to passthrough the liquid crystal layer 8, too, thereby leading to nocontribution to display contrast formation.

However, such components give only a small reduction in contrast becauseblocking by the polarization plate 10 is effected.

Besides, in the first embodiment, illumination light L2A passes throughthe polarization plate 10 twice before emitting to the outside. On theother hand, in the present embodiment, illumination light L2A passesthrough the polarization plate 10 only once before emitting to theoutside. This reduces loss to improve display contrast and brightness.

Referring to FIG. 7, illustrated is a liquid crystal display inaccordance with another embodiment of the present invention. Elementsused in common to the arrangements shown in FIG. 8, FIG. 9, FIG. 1, FIG.2 or FIG. 6 are indicated by common references, with repeateddescriptions being simplified.

A liquid crystal display 50 comprises a surface light source device ofside light type disposed behind a liquid crystal layer 8 (as viewed fromthe viewing side). According to an arrangement different from thearrangement of the previous embodiment, the surface light source deviceis incorporated in a liquid crystal display panel to provide anauxiliary front-lighting means. Display screen is viewed from above inFIGS. 7.

In order from the back side (from the bottom in FIG. 7), laminatedlydisposed are a reflection plate 23, a first polarization plate 6, aguide plate 25, a glass substrate 7, a liquid crystal layer 8, a glasssubstrate 9 and a second polarization plate 10. An important feature ofthe arrangement is that the guide plate 25 is interposed between thesecond polarization plate 10 and the liquid crystal cell (in detail, theglass substrate 7).

Transparent electrodes are formed on surfaces of the glass substrates 7and 9, respectively, providing transparent electrode patterns betweenwhich a liquid crystal material is interposed and sealed to form theliquid crystal layer 8. Each liquid crystal cell rotates a polarizationplane of light transmitting therethrough depending on voltage applied tothe transparent electrodes.

Each of the polarization plates 6 and 10 permits a componentcorresponding to a certain direction of polarization plane (transmissionpolarization plane) to transmit exclusively. In general, thepolarization plates 6 and 10 are orientated so that the transmissionpolarization plane of the first polarization plate 6 is eitherperpendicular or parallel to that of the second polarization plate 10.

The reflection plate 23 is a member provided with scattering propertiesand high reflectivity with respect to light which is transmitted throughthe liquid crystal cells, having substantially no light transmissivity,which may be the same element as is employed in the embodiment of FIG.1.

On impinging of ambient light L1 to the liquid crystal display panelfrom above in FIG. 7, some components of the ambient light transmitthrough the polarization plate 10, the glass substrate 9, the liquidcrystal layer 8, the glass substrate 7, the guide plate 25 and thepolarization plate 6 to reach the reflection plate 23.

Quantity of light reaching the reflection plate 23 depends on factorsincluding directions of transmission polarization planes of thepolarization plates 6, 10 and the state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes).

The reflection plate 23 scatters and reflects ambient light L1.Substantially no light transmits through the reflection plate 23. Somecomponent of the scattered and reflected light transmits through thepolarization plate 6, the guide plate 25, the glass substrate 7, theliquid crystal layer 8, the glass substrate 9 and the polarization plate10 in order, being emitted toward the outside to contribute todisplaying.

Quantity of light emitting toward the outside vanes depending on factorsincluding directions of transmission polarization planes of thepolarization plates 10, 6 and the state of the liquid crystal layer 8(depending on voltage applied to the transparent electrodes). Accordingto this principle, bright-dark distribution is formed to provide imagesto be viewed.

When a fluorescent lamp 14 is turned on, primary light is introducedinto the guide plate 25 through an opening of a reflector 15 and an endface 25A of the guide plate. The guide plate 25 may be the same asemployed in the embodiment of FIG. 1 or the embodiment of FIG. 6.Attention should be paid to a fact that a back face 25B is directed tothe liquid crystal cell (in detail, the glass substrate 7) and anillumination output face 25C is directed to the polarization plate 6.

The back face 25B provides a light control face 25D which includes anemission promoting face for helping emission from an illumination outputface 25C. The illumination output face 25C is a specular face havingsubstantially no scattering power.

Illumination light L2 introduced into the guide plate 25 goes back andforth repeatedly between the back face 25B (light control face 25D) andthe illumination output face 25C on the way of propagation. This processinvolves gradual emitting from the back face 25B (light control face25D) and the illumination output face 25C.

Some of the outputted illumination light from the illumination outputface 25C transmits through the polarization plate 6 to reach thereflection plate 23. The proportion of light reaching the reflectionplate 23 depends on a direction of the transmission polarization planeof the polarization plate 6.

The reflection plate 23 scatters and reflects the outputted illuminationlight. Substantially no light transmits through the reflection plate 23.Some component of the scattered and reflected light transmits throughthe polarization plate 6, the guide plate 25, the glass substrate 7, theliquid crystal layer 8, the glass substrate 9 and the polarization plate10 in order, being emitted toward the outside to provide light L2Acontributing to displaying.

The quantity of light emitting toward the outside varies depending onfactors including the directions of transmission polarization planes ofthe polarization plates 10, 6 and the state of the liquid crystal layer8 (depending on voltage applied to the transparent electrodes):According to this principle, bright-dark distribution is formed toprovide images to be viewed.

The light control face 25D is provided with emission promotingproperties to promote emission of illumination light L2. This emissionpromoting properties tend to be at first stronger according to distancefrom the incidence face 25A and to be weaker thereafter. The reason whysuch reduced emission promoting properties are assigned to around adistal end is that a distal end face produces reflection light whichwill cause illumination output to be increased.

The present embodiment also employs emission promoting regions which aredistributed on the back face 25B according to a light control pattern.The light control pattern is designed so that the above tendency isrealized. The light control pattern illustrated in FIG. 3a may beemployed in the present embodiment also. Repeated descriptions on thelight control pattern shown in FIG. 3a are omitted.

Comparing the present embodiment with the embodiments of FIGS. 1 and 6,the followings will be understood. As previously discussed, it isimpossible to achieve perfect suppression of illumination light L2Bwhich is directly emitted from the back face 25B of the guide plate 25.

However, the present embodiment is capable of restraining white-ishbackground which would lead to reduction in display contrast fromappearing, because the liquid crystal cell is arranged at the outside ofthe guide plate 25.

Some of illumination light L2B is reflected by elements such as theglass substrates 7, 9 before reaching the polarization plate 10,returning to the guide plate 25. Such illumination light will be able tohave a chance to be emitted from the illumination output face 25C. Thisresults in less quantity of light which is consumed for formingwhite-ish background and in much quantity of light which contributes toforming display contrast, as compared with the embodiments of FIGS. 1and 6.

The light control face 25D promotes inside propagating light L2 to beemitted, thereby causing the illumination output face 25C to emit suchlight as represented by illumination light L2AA in addition to frontalemission. This illumination light L2AA is greatly inclined with respectto a propagation direction of illumination light inside the guide plate25 (from the right hand to the left hand, in FIG. 7).

In the above described embodiments of FIGS. 1 and 6, effectiveutilization of such oblique illumination light L2AA is hardly expected.On the other hand, the present embodiment permits some of such light totransmit through the polarization plate 6 and then to be scattered andreflected by the reflection plate 23. As a result, some of such lightproceeds along paths similar to illumination light L2A, contributing todisplay. This will also leads to improvement in brightness and contrast.

None of the above embodiments aim to be limiting to the scope of thepresent invention. For instance, the following modifications fall withinthe scope of the present invention.

(a) In the above embodiments, each light control pattern is composed ofa great number of circular dot regions, respectively. However, thisimposes no limitation on the present invention. Other various shapessuch as rectangular may be employed for regions forming a light controlpattern.

(b) These fine regions may be other than roughened surface produced bymat-processing. For example, light scattering ink containing a pigmentsuch as magnesium carbonate or titanium oxide may be selectively appliedto a surface of a guide plate to provide a light control pattern.

(c) Further, micro-lens-like shapes may be formed on a surface of aguide plate by means of metal molding, printing or others to provide alight control pattern.

(d) In the above embodiments, each light control pattern is arrangedwith irregularity. However, this imposes no limitation on the presentinvention. So far as Moire fringes are prevented practically enough,arrangements with regularity are employable.

(e) In the above embodiments, the fine regions forming light controlpattern have almost invisible dimensions. However, this imposes nolimitation on the present invention. Unless displaying is preventedpractically, sizes larger than the above stated are employable.

As to the covering rate, it may be designed flexibly according to thesituation. For instance, each light control pattern is viewed acrosssome additional elements such as the polarization plate in theembodiments of FIGS. 6 and 7. Accordingly, fine regions with sizeslarger than the above stated data may be employed to form light controlpatterns.

(f) In the above embodiments, each glass substrate is a member and eachpolarization plate is another member. However, this imposes nolimitation on the present invention.

These glass substrates and polarization plates may be unified. Forexample, unifying of glass substrate and polarization plate may beapplied behind the liquid crystal layer in the embodiment of FIG. 6, infront of the liquid crystal layer in the embodiment of FIG. 7 and at theboth sides of the liquid crystal layer in the embodiment of FIG. 1,respectively.

(g) In the above-described embodiments, transparent guide plates areemployed. However, this imposes no limitation on the present invention.Various materials may be employed so far as effects are performed incommon with the above-described embodiments. For instance, a scatteringguide plate provided with inside scattering power may be employed.

(h) The surface light source devices in the above-described embodimentsare supplied with primary light from rod-shaped light sources(fluorescent lamps). However, this imposes no limitation on the presentinvention. For example, a primary light source provided with anarrangement including a plurality of point-like light source elementssuch as LEDs may be employed.

What is claimed is:
 1. A surface light source device of side light type applied to auxiliary lighting in a liquid crystal display including a reflection plate and a liquid crystal display panel having a front face for being supplied with ambient light and a back face along which said reflection plate is disposed, comprising: a guide plate; and a primary light source to supply primary light to an incidence end face provided by a minor face of the guide plate, said guide plate having major faces to provide a back face and an illumination output face, wherein said illumination output face is provided with no light scattering property and supplies light toward said front face of said liquid crystal display panel; said back face provides a light control face provided with emission promoting properties which help light propagating within the guide plate to escape from said illumination output face by substantially scattering the light.
 2. A surface light source device of side light type in accordance with claim 1, wherein said light control face comprises a number of dot-like fine regions to promote emission.
 3. A surface light source device of side light type if accordance with claim 2, wherein said dot-like fine regions are arranged with irregularity.
 4. A surface light source device of side light type if accordance with claim 2, wherein said dot-like fine regions are roughened partial regions in said back face.
 5. A surface light source device of side light type if accordance with claim 3, wherein said dot-like fine regions are roughened partial regions in said back face.
 6. A surface light source device of side light type in accordance with any one of claims 2, 3, 4 or 5, wherein said fine regions have such dimensions respectively as to make the fine regions almost invisible.
 7. A surface light source device of side light type in accordance with any one of claims 2, 3, 4 or 5, wherein at least a part of said back face is provided with emission promoting property which tends to be stronger according to distance from said incidence end face.
 8. A surface light source device of side light type in accordance with claim 6, wherein at least a part of said back face is provided with emission promoting property which tends to be stronger according to distance from said incidence end face.
 9. A liquid crystal display comprising: a liquid crystal display panel having a front face for being supplied with ambient light and a back face; a reflection plate disposed along said back face of said liquid crystal display panel, and a surface light source device of side light type for auxiliary lighting, wherein said surface light source device of side light type comprises a guide plate and a primary light source to supply primary light to an incidence end face provided by a minor face of the guide plate and said guide plate having major faces to provide a back face and an illumination output face; wherein said illumination output face is provided with no light scattering property and supplies light toward said front face of said liquid crystal display panel; and, said back face provides a light control face provided with emission promoting properties which help light propagating within the guide plate to escape from said illumination output face by substantially scattering the light.
 10. A liquid crystal display in accordance with claim 9, wherein said light control face comprises a great number of dot-like fine regions to promote emission.
 11. A liquid crystal display in accordance with claim 10, wherein said dot-like fine regions are arranged with irregularity.
 12. A liquid crystal display in accordance with claim 10, wherein said dot-like fine regions are roughened partial regions in said back face.
 13. A liquid crystal display in accordance with claim 11, wherein said dot-like fine regions are roughened partial regions in said back face.
 14. A liquid crystal display in accordance with any one of claims 10, 11 or 12, wherein said fine regions have such dimensions respectively as to make the fine regions almost invisible.
 15. A liquid crystal display in accordance with any one of claims 9, 10, 11, 12 or 13, wherein at least a part of said back face is provided with emission promoting property which tends to be stronger according to distance from said incidence end face.
 16. A liquid crystal display in accordance with claim 14, wherein said back face is provided with emission promoting property which tends to be stronger according to distance from said incidence end face.
 17. A liquid crystal display comprising: a liquid crystal layer being supplied with ambient light from a front side of the liquid crystal layer; a reflection plate disposed at a back side of said liquid crystal layer; a first polarization plate interposed between said liquid crystal layer and said reflection plate; a second polarization plate disposed at the front side of said liquid crystal layer; and, a surface light source device of side light type for auxiliary lighting which comprises a guide plate and a primary light source to supply primary light to an incidence end face provided by a minor face of the guide plate, said guide plate having major faces to provide a back face and an illumination output face, wherein said illumination output face is provided with no light scattering property, said back face provides a light control face provided with emission promoting properties which help light propagating within the guide plate to escape from said illumination output face by substantially scattering the light; and said guide plate being interposed between said liquid crystal layer and the second polarization plate so that said illumination output face is directed to the front side of said liquid crystal layer.
 18. A liquid crystal display in accordance with claim 17, wherein said light control face is provided with a great number of dot-like fine regions to promote emission.
 19. A liquid crystal display in accordance with claim 18, wherein said dot-like fine regions are arranged with irregularity.
 20. A liquid crystal display in accordance with claim 18, wherein said dot-like fine regions are roughened partial regions in said back face.
 21. A liquid crystal display in accordance with claim 19, wherein said dot-like fine regions are roughened partial regions in said back face.
 22. A liquid crystal display in accordance with any one of claims 18, 19, 20 or 21, wherein said fine regions have such dimensions respectively as to make the fine regions almost invisible.
 23. A liquid crystal display in accordance with any one of claims 18, 19, 20 or 21, wherein at least a part of said back face or said illumination output face is provided with emission promoting property which tends to be stronger according to distance from said incidence end face.
 24. A liquid crystal display in accordance with claims 22, wherein at least a part of said back face or said illumination output face is provided with emission promoting property which tends to be stronger according to distance from said incidence end face.
 25. A surface light source device as recited in claim 1, wherein the back face substantially scatters the light and substantially reflects the light.
 26. A surface light source device as recited in claim 1, wherein the substantial scattering is effectuated by a scattering unit designed to scatter light. 